Structure of MgSiO3 glass at pressures near the Earth’s core-mantle boundary

Figure caption: Structure factor S(Q) of MgSiO<sub>3</sub> glass up to 111 GPa, displayed by a vertical offset of +0.42 for B and +0.48 for C and D.
Figure caption: Structure factor S(Q) of MgSiO3 glass up to 111 GPa, displayed by a vertical offset of +0.42 for B and +0.48 for C and D.

Knowledge on the structure and properties of silicate magma under extreme pressure plays an important role in understanding the nature and evolution of Earth’s deep interior.  However, such information is scarce owing to experimental challenges.  Using a recently developed double-stage Paris-Edinburgh press, combined with the multi-angle energy dispersive X-ray diffraction, a team has measured structures of MgSiO3 glass up to 111 GPa. The results revealed direct experimental evidence of a structural change in this glass at >88 GPa. The structure above 88 GPa is interpreted as having the closest edge-shared SiO6 structural motifs similar to those of the crystalline MgSiO3 postperovskite, with densely packed oxygen atoms. The pressure of the structural change is broadly consistent with or slightly lower than that of the bridgmanite-to-postperovskite transition in crystalline MgSiO3. Considering similarities in pressure-induced structural changes between silicate melts and glasses, a similar ultrahigh-pressure structural change may occur in MgSiO3 melts in the deep lower mantle. These results suggest that the ultrahigh-pressure structural change may occur in silicate melts above the CMB, with significant densification and potentially profound influence on the dynamics of melt storage and circulation in the Earth’s deep interior. More in Kono et al., ‘Pressure-induced structural change in MgSiO3 glass at pressures near the Earth’s core-mantle boundary’, PNAS, www.pnas.org/cgi/doi/10.1073/pnas.1716748115.